1. Field of the Invention
The present invention relates generally to a printed circuit board, and more particularly, to a method for fabricating a printed circuit board for which wirings of printed circuit patterns are printed on both sides of the board, and connected to each other.
2. Description of the Related Art
Currently, small electronic devices requiring portability, such as a portable terminal and a portable video game, are increasing in popularity and electric home appliances are decreasing in size. However, these devices also are providing cutting-edge functions, thus causing a sharp increase in integration of their electronic parts.
In these types of small electronic devices, i.e., those that need wiring work in a small space, flexible printed circuit boards made of polyimide resin are often used, because the flexible printed circuit boards can be easily reduced in thickness and freely transformed into various forms. Further, the polyimide resin is superior in thermal resistance and electrical insulating properties.
FIG. 1 is a cross-sectional view illustrating a flexible printed circuit board found in the prior art.
Referring to FIG. 1, a conventional printed circuit board 10 has wirings 13 formed on both sides of a base film 11, and the wirings 13 are conductively connected to each other through via-holes 15 as occasion arises. In addition, cover layers 17 are formed on the wirings 13 using, for example, adhesives 19, to protect the wirings 13 formed on both sides of the base film 11.
On the conventional printed circuit board 10, copper foils forming the wirings 13 are stacked on one or both sides of the base film 11, and after the stacking process, the via-holes 15 are made in necessary positions. Thereafter, to form the wirings 13 in a desired pattern with the stacked copper foils, a Dry Film photoResist (DFR) is applied, and the remaining DFR excluding the regions corresponding to the wiring shapes of the desired pattern is removed through an exposure and development process. Thereafter, the resulting DFR is subject to etching to remove the copper foils in unnecessary regions, thereby completing the wirings 13.
Finally, a conductive paste is poured into the via-holes 15 formed in the base film 11 to conductively connect the wirings 13 formed on both sides of the base film 11.
However, the method of fabricating the conventional printed circuit board has a complicated fabrication process including the process of exposing and developing the DFR and the etching process for removing the copper foils in unnecessary regions, thus increasing the fabrication cost. This fabrication cost then likely increases the cost of the device for the users.
Further, the conductive paste injected into the via-holes should be adhered closely to wall surfaces of the via-holes. Undesirably, however, the injected conductive paste may not be well adhered to the wall surfaces of the via-holes, reducing reliabilities of the products. In particular, when the flexible printed circuit boards are crookedly transformed by the external forces during their wiring or transport process, the conductive pastes, which were hardened without being adhered closely to the wall surfaces of the via-holes, may be cracked, increasing the failure rate.